Microfabrication and nanofabrication processing provide commercially attractive techniques for patterning structures comprising active and passive components of electronic, optical and optoelectronic devices and device arrays. The spectrum of available techniques include photolithography, soft lithography (e.g., contact printing), electron beam direct writing, and photoablation patterning methods. These techniques provide fabrication pathways for making, assembling and integrating large numbers of nanosized and or microsized structures comprising a variety of useful functional materials. Available techniques also provide a high degree of deterministic control over the physical dimensions and spatial arrangement of patterned structures and are compatible with processing a range of substrate materials, morphologies, and sizes. Microfabrication and nanofabrication fabrication techniques have been developed capable of accessing a range of useful device component and device array geometries, including complex three-dimensional multilayer thin film configurations.
Given the capabilities of microfabrication and nanofabrication processing, substantial research activities are currently directed toward enhancing application of these techniques for high resolution, dense patterning of large substrate areas. Substantial motivation for large area patterning applications originates from the rapidly expanding field of large area integrated electronics. Advances in materials strategies and fabrication techniques for thin film based electronics, for example, have played an essential role in the commercial development of a large area, dense integrated circuits and microelectronic systems having important applications for electronic displays, thin film photovoltaic systems, light emitting diode arrays, medical imaging systems and sensing technologies. Critical to the continued development and commercialization of this field, however, is the development of high through-put, low cost microelectronic and nanoelectronic processing techniques capable of large area and high resolution patterning and also capable of effective integration with existing device fabrication strategies and infrastructure.
Photolithography in combination with thin film deposition and etching processing provides a highly developed microfabrication platform for making large numbers (e.g., 10s of millions) of microstructures or micro-devices in the fabrication of a wide range of devices, including large area electronic devices. For example, this combination of processing techniques provides for the fabrication, assembly and integration of diverse classes of: (i) functional materials, including inorganic and organic semiconductors, dielectrics, conductors and optical active materials; (ii) active and passive device components, including transistors, capacitors, resistors, connectors and electrodes; and (iii) functional devices such as diodes, light emitting devices, and photovoltaics. These methods are compatible with processing diverse substrate materials, ranging from semiconductors, glasses, ceramics and polymer substrates, and processing functional substrates, such as printed circuit boards (PCB), display panels, and multi-chip modules. Photolithography techniques also enable patterning over a useful range of substrate areas ranging from a few square microns in microelectromechanical systems (MEMS) to a few centimeters in integrated circuits and up to few square meters in displays. As a result of these demonstrated performance attributes, photolithography is currently the most widely adopted microfabrication technique for the commercial fabrication of electronic and optoelectronic devices and device arrays.
In conventional photolithography, a substrate is coated or covered with a layer of photoresist (PR), for example via spin coating, linear coating, inkjet printing or vapor deposition methods. The PR is typically baked so as to stabilize the layer for subsequent processing steps. In subsequent photo-processing steps, the stabilized PR layer is illuminated with electromagnetic radiation having a selected, non-uniform spatial distribution of radiant energies. Typically, this is achieved using an optical mask have opaque features on a transparent background to generate an illumination pattern that corresponds to the optical mask geometry. The PR undergoes chemical changes in illuminated areas such that subsequent PR processing or development generates patterns in the photoresist corresponding to whether or not the region was masked. For example, exposure of a PR layer to electromagnetic radiation may cause polymer comprising the PR in the illuminated regions to become dissociated or cross-linked due to absorption of the radiant energy. In a subsequent development step, the illuminated PR is removed, for example by dissolution by developer, for negative PR materials, or the PR that is not illuminated is removed for positive PR materials. This developing processing step, therefore, provides a means of generating a pattern of exposed regions of the underlying substrate corresponding to illuminated or non-illuminated regions of the PR removed upon developing. Subsequent processing steps may include deposition and/or etching of the exposed regions of the substrate, wherein the PR functions as a mask to provide processing spatially restricted to the exposed regions of the substrate useful for patterning. The remaining PR on the material may be removed by stripper, resulting in a pattern of structures or features on the substrate. Two primary photolithography methods are currently in wide use for making microelectronic and optoelectronic devices: (i) contact printing methods, and (ii) projection imaging methods.
In contact printing methods, an optical mask is provided in mechanical contact with the PR layer is deposited on a surface of the substrate undergoing processing. This mask configuration enables patterning of the PR upon subsequent illumination and developing processing steps. There are significant disadvantages associated with this mask configuration, however, which impede commercial implementation of contact printing methods for applications requiring high resolution patterning. First, mechanical contact between mask and PR can introduce unwanted changes in the optical transmission properties of the mask, thereby undermining the pattern fidelity and resolution of the patterning achieved using this photolithographic technique. For example, mechanical contact between the PR layer may introduce optical defects and distortions capable of decreasing transmission of the illumination in specific portions of the mask, thereby resulting in pattern defects. Second, repeated mechanical contact between mask and PR layer results in mask degradation over time, thereby introducing pattern defects and limiting effective mask lifetime.
Given the disadvantages inherent to contact printing systems, projection imaging methods are more commonly used photolithographic patterning techniques for fabrication of integrated circuits, displays and other microelectronic products. These systems utilize an optical configuration avoiding direct contact of an optical mask with the PR layer to be patterned. Projection imaging systems consist of three principle components: (i) a light source for generating light having a desired wavelength, radiant energy and uniformity; (ii) optical components which transmit the light from source to substrate; and (iii) a stage part which holds substrate and moves exactly for pattern alignment. Optical components for light transmission in projection imaging photographic systems typically include a plurality of lenses to achieve uniform and exact alignment to the substrate. Two kinds of systems are principally used in projection imaging methods: scan projection-type systems and stepping-type projection systems.
In scan-type projection systems, illumination is scanned on the entire substrate area by moving the optical parts or moving the substrate. An advantage of scan-type projection is that it is capable of efficiently and simultaneously illuminating large substrate areas, thereby providing a fabrication platform compatible with some large-area patterning device applications, such as fabricating display devices or integrated circuits requiring patterning over large substrate areas. A disadvantage of scan-type projection methods, however, is the significant price of scan-type projection systems capable of large area patterning. For example, the price of critical optical components, such as lenses, can become prohibitively for large area scan-type projection systems, thus placing practical limitations on commercial implementation of this technique for some important applications.
In stepping-type projection systems, a substrate area to be patterned is divided into discrete segments that are successively and independently illuminated segment by segment. Multi-step processing providing a series of repeated patterns on a substrate is useful for some large area microelectronic device fabrication applications. The multi-step nature of processing in stepping-type projection methods, however, decreases total throughput and makes this technique susceptible to processing errors related to the alignment and registration of independently processed substrate segments. FIG. 1 provides a schematic diagram illustrating mechanical alignment and registration problems which can arise in stepping-type projection photolithography methods. As shown in this Figure, the substrate 41 is illuminated by three stepping shots 42, 43, and 44. If there is a misalignment (schematically shown as 45 in FIG. 1) in the first shot 42 and the second shot 43, the illumination energy (shown on the Y-axis 46 of the plot provided in FIG. 1) is doubled where the illumination is duplicated. As also shown in FIG. 1, however, this misalignment also results in a non-illuminated region between the second shot 43 and third shot 44, wherein the energy of illumination is zero. The patterns on the border area could be over-developed or under-developed because of this difference in illumination energy. In the case of fabrication TFT-LCD display devices, this non-uniform pattern in the substrate, as also known as a stitching problem, can result in unwanted non-uniform images. Substantial research attention is currently directed at addressing this problem.
To provide an alternative to photolithographic processing, complementary focused beam laser direct writing techniques, also commonly referred to as direct laser ablation methods, have been developed, and are currently used in some important microfabrication applications. In these techniques, direct patterning of selected areas on a substrate is achieved via illumination with focused laser electromagnetic radiation without the need for photoresist processing. Illumination is typically realized through an optical mask in a raster scanning fashion to expose specific portions of the substrate to focused laser electromagnetic radiation. The large radiant energies provided by the focused laser beam, initiate chemical reactions and physical processes resulting in ablative removal of material capable of absorbing laser electromagnetic radiation. Direct laser ablation methods provide a number of benefits including eliminating the need for a PR coating system, developing system, bake system and stripping system. Total fabrication times provided by laser ablation systems are also low due the direct nature of this patterning technique. For some patterning applications, these benefits significantly reduce the number of fabrication steps, thereby achieving a net reduction in fabrication costs and time. Implementation of direct laser ablation methods for making large area microelectronic applications, is currently restricted, however, due to significant limitations in the classes of materials that can be effectively patterned via laser ablation.
A number of studies have demonstrate the ability to directly pattern indium tin oxide (ITO) thin films via laser ablation methods. [See, e.g., O. Yavas and M. Takai, “Effect of Substrate Absorption on the Efficiency of Laser Patterning of Indium Tin Oxide Thin Films”, Journal of Applied Physics, Vol. 85, (1999), pp. 4207-4212; O. Yavas and M. Takai, “High-Speed Maskless Laser Patterning of Thin Films for Giant Microelectronics”, Journal of Applied Physics, Vol. 38 (1999), pp. 7131-7134; and C. Molpeceres, S. Lauzurica, J. L. Ocana, J. J. Gandia, L. Urbina and J. Carabe, “Microprocessing of ITO and a-Si thin films using ns laser sources”, Journal of Micromechanics and Microengineering, 15 (2005) 1271-1278;]. As ITO is an important transparent conductor material useful in a range of device fabrication applications, these studies identify a potential to use direct laser ablation methods for making important large area microelectronic devices, such as flat panel display devices, photovoltaic arrays and large area sensors. O. Yavas and M. Takai report observation of wavelength dependent patterning characteristics of ITO films via laser ablation, wherein illumination with high energy ultraviolet electromagnetic radiation (λ=262 nm) is reported as capable of generating electrically isolating lines having smooth etch groove in a thin ITO film. These authors suggest the importance of strong absorption of the laser light by the substrate for the improved pattern morphology observed upon ultraviolet light irradiation. C. Molpeceres et al. also report effective ultraviolet laser ablation of thin films of indium tin oxide at λ=248 nm and 355 nm. These authors report fabrication of narrow ablation grooves with widths varying between 6 and 30 microns with processing velocities ranging from 1 mm s−1 to 20 mm s−1.
Despite the demonstrated capability of direct patterning of ITO films via laser ablation, these methods are susceptible to drawbacks which significantly hinder its commercial implementation in various device fabrication applications. First, patterning via direct ablation may result in the build up of ITO ablation debris on the substrate surface that can cause electrical shorts between patterned ITO features. Such ablation debris caused shorts can negatively affect device performance, for example, by causing a point or line defect in a microelectronic display device. Second, direct laser ablation of thin ITO films can result in unwanted exposure of the substrate to significant radiant energies of laser electromagnetic radiation. For application requiring ITO patterning on substrates prepatterned with electronic device components, such as thin film transistor components of microelectronic display devices, this exposure can result in unwanted changes in the electronic, optical and/or mechanical properties of device components underlying the patterned ITO layer. These changes can result in significant degradation of overall device capabilities and performance. Third, it is not clear that direct ablation methods are capable of providing the pattern linearity necessary for some device fabrication applications, such as In-Plane Switch (IPS) mode TFT-LCD systems, requiring very high resolution patterning of ITO components.
It will be appreciated from the foregoing that that there is currently a need for microfabrication and nanofabrication processing methods capable large area patterning of substrates to enable the rapidly developing class of large area integrated electronic systems. Processing methods are needed capable of generating high resolution, dense patterns of structures providing active and passive components of a variety of microelectronic devices. It will also be appreciated that a need exists for new methods of patterning important functional materials, such as indium tin oxide, for display systems, such as thin film transistor—liquid crystal display systems (TFT-LCD), organic light emitting displays (OLED) and plasma display panels (PDP).